fix init koptcontext with all zero bbox
fix init koptcontext with all zero bbox

houqp/koreader,poire-z/koreader,mihailim/koreader,mwoz123/koreader,ashang/koreader,apletnev/koreader,NickSavage/koreader,chrox/koreader,robert00s/koreader,ashhher3/koreader,chihyang/koreader,koreader/koreader,Frenzie/koreader,poire-z/koreader,NiLuJe/koreader,NiLuJe/koreader,pazos/koreader,koreader/koreader,Markismus/koreader,Hzj-jie/koreader,Frenzie/koreader,frankyifei/koreader,noname007/koreader,lgeek/koreader